In the field of x-ray imaging, two detection modes are commonly employed. A first mode, called the direct detection mode, consists in using a matrix array of photodetectors, each photodetector being able to convert the x-rays that it absorbs into electric charge. A second mode, called the indirect mode, consists in initially converting the x-rays into visible photons, via a scintillator, and then in using a matrix array of photodetectors to convert the produced visible photons into electric charge. The invention relates to a matrix array of pixels for indirect detection of x-rays, each pixel being composed of at least one thin film transistor (TFT) coupled to an organic photodetector. In each of the pixels, a transistor is commonly connected to a first electrode of an organic photodetector.
A layer suitable for the photo-conversion of light is commonly deposited on the first electrode. This layer may for example be organic and include a nanostructured mixture of p-type and n-type semiconductors (Li, G., Shrotriya, V., Huang, J., Yao, Y., Moriarty, T., Emery, K., & Yang, Y., 2005, High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nature materials, 4(11), 864-868). A top electrode is then deposited on the photo-conversion layer.
FIG. 1 schematically illustrates the structure of an organic photodiode according to the prior art. The stack for example includes a transparent substrate (made of glass, polyethylene naphthalate (PEN) or polyethylene terephthalate (PET)). This substrate is covered with a transparent metal electrode (for example made of indium tin oxide (ITO)) and then a hole-collecting layer (HCL) that is able to collect holes during an illumination, this layer for example being made of poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). These layers are covered by a layer suitable for the photo-conversion, which is called the active layer and produced such as described above. Lastly, the active layer is covered with an electron-collecting layer (ECL), which is for example made of aluminum. In the example illustrated in FIG. 1, the photodiode is illuminated through the transparent substrate: this illuminating mode, and the structure of the photodiode, are said to be direct. Layers for transporting the charge carriers are electrically connected to the layers for collecting the charge carriers (HCL and ECL). In the direct structure illustrated in FIG. 1, the transport of the holes is achieved via an ITO layer and the transport of the electrons is achieved via an aluminum layer.
FIG. 2 schematically illustrates an organic photodiode of so-called inverted structure, according to the prior art. The illustrated photodiode includes a transparent substrate covered with a transparent electron-transporting layer (ETL), itself covered with a transparent electron-collecting layer (ECL). These layers are covered with an active layer and a hole-collecting layer (HCL). The HCL is for example covered by a silver layer, the function of which is both to allow the transport of the holes (HTL) and to reflect incident light coming from the substrate. The ECL is for example made of zinc oxide (ZnO) or of titanium oxide (TiOx) and the HCL is for example made of PEDOT: PSS or of a metal oxide such as molybdenum oxide, tungsten oxide or vanadium oxide. A structure of this type is disclosed by Jeong, J. et al., Inverted Organic Photodetectors With ZnO Electron-Collecting Buffer Layers and Polymer Bulk Heterojunction Active Layers, Selected Topics in Quantum Electronics, IEEE Journal of, 20(6), 130-136.
In these two direct or inverted photodiode structures, light may be absorbed by the various layers, and in particular by the top electrode and/or the bottom electrode.
It would be desirable to manufacture a matrix array of inverted organic photodiodes such as described above for medical imaging applications. This type of imaging requires very low detection thresholds. One of the ways of achieving a low detection threshold is to limit or even to suppress the dark current of a photodiode, i.e. the residual current of the photodiode in the absence of illuminating light, when the photodiode is biased. If the work function of the material of the electron-collecting layer is too high, it promotes parasitic injection of holes from this layer into the donor of the active layer. One of the solutions of the prior art is to make a bottom electrode (electrode in contact with the substrate) from a metal the work function of which is lower than that of commonly used materials (generally ITO). For example, aluminum and chromium have a work function lower than ITO. These materials have the drawback of being unstable in the presence of air because they are easily oxidizable.
This technical problem may be partially solved, as described by Jeong, J. et al. by using an electron-collecting layer that is interstitial between the bottom electrode and the active layer, and the role of which is to decrease the work function of the material making contact with the active layer: zinc oxide (ZnO) may be used for this purpose. The ZnO used is a semiconductor: its use in a whole-area deposition (without a pattern-defining lithography step) is technically problematic, because leakage currents may be generated between the various pixels of a photodiode matrix array. A defective pixel, for example in the case of a work function that is accidentally unsuitable for the active layer, may induce leakage currents in all of the neighboring pixels and make the zone of pixels that surrounds it unsuitable for imaging. A lithography step allowing the electron-collecting layer to be etched in order to separate the various pixels electrically could be one technical solution. This step is undesirable in a manufacturing process in which having too many required lithography steps in succession compromises the production of the device and/or its manufacturing yield.